A foil-blowing apparatus of the above-described type generally comprises an annular nozzle defined by an inner and/or outer closed annular lip, frequency referred to hereinafter as an annular gap nozzle lip, of metal from which a thermoplastified synthetic resin emerges, as well as a device for blowing the foil tube thus formed.
The blown foil balloon is continuously flattened to form a web of the foil which can be rolled up in the form of a double-layer web or flattened tube, or with appropriate slitting in the tube, in the form of two single-layer web.
Thickness errors in the plastic foil arise because of various effects. For example, such errors can arise where the structural elements forming the annular-gap orifice are not perfectly circular or are not ideally concentric with one another as a result of fabrication problems. Thickness errors can also arise because of temperature gradients in the foil tube, different mass distributions in the extruding of material from the orifice and like effects.
Thickness error correction is desirable because the further processing of plastic foils generally requires foils with narrow thickness tolerances, i.e. high thickness uniformity across the width and length of the web.
For example, should there be thickness variations, the foil cannot be satisfactorily wound up into a coil or roll or will be wound into a roll which is irregular and thus deviates from a cylindrical contour to a point that the roll cannot be handled easily.
It will be understood, of course, that thickness correction in the region of the orifice will have a significant effect on the thickness of the blown foil and thus it is desirable to effect such a thickness correction immediately upstream of the expansion of the tube to form the foil. The enable a thickness correction, a corresponding measurement of the thickness of the blown foil, in its balloon or bubble, or upstream of the coiling of the foil, or even by monitoring the contour of the coil, can be effected.
In earlier processes (see German patent documents DE 29 50 003 and DE 30 02 903, for example,) the thickness-error correction utilizes temperature-dependent rheological properties of the foil tube. For example, a thickness correction can result from localized or regional heating or cooling in the region of the nozzle. For this purpose, the blowing head in the region of the nozzle may be subdivided into a multiplicity of controlled temperature sectors which are controlled in response to the measurement of a thickness-error profile via a computer.
While this technique is satisfactory in correcting thickness errors, it has drawbacks because it affects detrimentally the optical quality of the foil which is produced.
Clearly, therefore, improvements are required in correction of thickness errors to avoid this drawback.
It has been recognized that it is also necessary, in the case where there may be residual errors, to distribute these errors throughout the periphery of the foil so as to avoid nonuniformity contours of the coil. This can be achieved according to prior art techniques by either imparting to the blowing head a periodic angular oscillation and/or by correspondingly moving the flattened device periodically. As a result, thickness errors will not accumulate at one part of the coil to produce an irregular cooling pattern.